The present invention relates to a process for the manufacture of alkanes in the presence of a supported organometallic compound which employs in particular an alkane cross-metathesis reaction.
Alkanes are generally products which are difficult to convert because of their chemical inertia. Nevertheless, the conversion of alkanes into other alkanes is known. Hydrogenolysis reactions, which consist of cleavage or opening reactions of a carbon-carbon bond by hydrogen, are known, for example. Isomerization reactions, which convert an alkane into one of its isomers, for example n-butane into isobutane, are also known. All these reactions are generally carried out at relatively high temperatures and in the presence of catalysts based on metals, in particular on transition metals, in the bulk form or in the form of films or alternatively in the form of metal particles deposited on inorganic supports essentially based on metal oxide. Thus, for example, the catalyst can be of the following type: nickel black, Ni/SiO2, platinum black, Pt/SiO2, Pd/Al2O3, or tungsten or rhodium film, optionally mixed with copper, tin or silver. With some metal catalysts, it was possible simultaneously to observe alkane homologation reactions, which consist of reactions which convert alkanes into higher alkanes. However, alkane homologation reactions are generally very minor reactions in comparison with the hydrogenolysis or isomerization reactions and their results are very poor.
Nevertheless, it remains the case that a process for the conversion of alkanes into their higher homologues would constitute a means for enhancing in value certain petroleum fractions, in particular the lightest fractions, such as the C4 or C5 fractions, or liquefied petroleum gas, also known as LPG. It is known that, as a general rule, alkanes of low molecular weight cannot be exploited to any great extent in chemistry or petrochemistry, other than as fuels, whereas heavier alkanes are often of greater commercial interest, such as, for example, to increase the octane number of engine fuels or alternatively to involve these heavier alkanes in thermal or thermal catalytic cracking or steam cracking reactions in order to manufacture, for example, olefins or dienes.
In this sense, Patent Application PCT/FR 97/01266 discloses a process for the conventional metathesis of alkanes in which at least one alkane is reacted with itself or several alkanes with one another in the presence of a solid catalyst comprising a metal hydride grafted to and dispersed over a solid oxide. Thus, a metathesis reaction is carried out in the presence of this metal hydride by cleavage and recombination of the carbon-carbon bonds, converting an alkane simultaneously into its higher and lower homologues. The reaction can be written according to the following equation (1):
2CnH2n+2xe2x86x92Cnxe2x88x92iH2(nxe2x88x92i)+2+Cn+iH2(n+i)+2xe2x80x83xe2x80x83(1)
where i=1,2,3, . . . nxe2x88x921 and n can range from 2 to 30 and even beyond.
The catalyst based on metal hydride comprises a transition metal chosen from those from groups 5 and 6 of the Table of the Periodic Classification of the Elements (as defined by IUPAC in 1991 and illustrated in xe2x80x9cHawley""s Condensed Chemical Dictionaryxe2x80x9d, 12th edition, by Richard J. Lewis, Sr., published by Van Nostrand Reinhold Company, New York, 1993), such as, in particular, tantalum, chromium or tungsten. The preparation of the catalyst comprises a stage of hydrogenation of an organometallic compound comprising a transition metal dispersed over and grafted to a solid oxide beforehand, so that the transition metal is reduced to an oxidation state lower than its maximum value, thus resulting in the metal hydride. However, like any conventional alkane metathesis reaction, in particular carried out in the presence of this metal hydride, higher and lower homologous alkanes are simultaneously manufactured, which limits the degree of freedom in the choice of alkane to be manufactured, since several types of alkanes are manufactured simultaneously.
A novel process for the manufacture of alkanes has now been found which makes use in particular of an alkane cross-metathesis reaction carried out in the presence of an organometallic compound. This novel process makes it possible to give an additional degree of freedom and a greater flexibility in the synthesis of alkanes. Thus, for the first time, it has been possible to demonstrate an alkane cross-metathesis reaction which consists essentially of cleavage and recombination reactions of carbon-carbon and carbon-metal bonds between at least one startng (initial) alkane and an organometallic compound comprising at least one hydrocarbon-comprising radical. The novel process can be employed under less severe conditions than the majority of processes known to date and is capable of easily and efficiently producing more desirable alkanes of economic value with various applications according to their molecular mass.
The subject-matter of the invention is therefore a process for the manufacture of alkanes, characterized in that it comprises, as main stage, a cross-metathesis reaction between at least one starting alkane (A) and one organometallic compound (B) fixed to an inorganic support and comprising a metal bonded to at least one hydrocarbon-comprising radical, which reaction results in the formation of at least one other alkane (C), which is a higher or lower homologue of the starting alkane (A), by cleavage of the hydrocarbon-comprising radical with the metal of the organometallic compound (B) and recombination of the said radical with at least one other radical originating from a cleavage of the starting alkane (A).
Thus, the process of the invention comprises an alkane cross-metathesis reaction which consists essentially of a stoichiometric reaction between the starting alkane (A) and the organometallic compound (B). The products resulting from the reaction include in particular at least the other alkane (C) and at least one other organometallic compound (D) having a different hydrocarbon-comprising radical from that of the compound (B).
By way of illustration, the alkane cross-metathesis reaction can be written according to at least one of the following two equations (2) and (3): 
in which equations Hxe2x80x94R1xe2x80x94R2 represents the starting alkane (A), H represents a hydrogen atom, R1 and R2 represent hydrocarbon-comprising radicals, and R3xe2x80x94M represents the organometallic compound (B) with M representing the metal bonded to R3, which itself represents a hydrocarbon-comprising radical.
Side reactions can take place in parallel with the alkane cross-metathesis reaction, in particular carbon-hydrogen bond cleavage and recombination reactions as described in the following equation (4): 
The invention thus makes it possible, by virtue of a cross-metathesis reaction, to convert the starting alkane (A) into at least one of its higher or lower homologues (C) by reactions of cleavage and of recombination with the hydrocarbon-comprising radical of the organometallic compound (B). More specifically, it makes use of cleavage reactions of carbon-carbon bonds of the starting alkane (A) and of cleavage reactions of the carbon-metal bond of the organometallic compound (B). The cleavage reactions are themselves followed by recombination reactions which form new carbon-carbon and carbon-metal bonds and thus result in at least one other alkane (C) and one other organometallic compound (D). This reaction is all the more surprising since only one conventional alkane metathesis reaction was known until now, converting, by catalytic reaction, an alkane simultaneously into one or more of its higher homologues and into one or more of its lower homologues according to, for example, the abovementioned equation (1). The alkane cross-metathesis reaction, in contrast to the conventional metathesis reaction, does not employ a catalyst, such as a supported metal hydride, but an organometallic compound which takes part as reactant in a stoichiometric reaction with an alkane.
One of the advantages of the cross-metathesis is that of being able to direct as desired the reaction towards the formation essentially of one or more desired alkanes. This can be obtained by virtue of the choice of the organometallic compound (B) and of its hydrocarbon-comprising radical which, by cleavage with the metal and recombination with another radical originating from the starting alkane, results directly in the desired alkane. In contrast, in conventional metathesis, at least two alkanes are formed simultaneously, one a higher homologue and the other a lower homologue, according to multiple cleavage and recombination reactions essentially produced at random on the starting alkane.
The cross-metathesis reaction as employed in the process of the present invention can be continued and accompanied by a conventional metathesis reaction, as represented schematically by the abovementioned equation (1). This is because it has been observed that higher and lower homologous alkanes can be formed simultaneously from the starting alkane. Furthermore, the decomposition product of the organometallic compound (B) resulting from the cross-metathesis is also capable of catalysing a conventional alkane metathesis reaction, such as that disclosed in Patent Application PCT/FR97/01266.
The starting alkane (A) employed in the process of the present invention can be a substituted cyclic or branched or linear alkane, for example a C2 to C80 alkane. It can be a matter in particular of C2 to C17 alkanes chosen, for example, from ethane, propane, isobutane, n-butane, isopentane, n-hexane, 2-methylpentane, 3-methylpentane and 2,3-dimethylbutane. The starting alkane can also be chosen from C18 to C80 alkanes, such as alkanes constituting waxes, in particular petroleum waxes, for example paraffin waxes (or macrocrystalline waxes) or microcrystalline waxes, and synthetic waxes, for example xe2x80x9cFischer-Tropschxe2x80x9d waxes or polyolefin waxes.
The starting alkane can be a linear or branched alkane corresponding to the general formula CnH2n+2 in which n is a number ranging from 2 to 80, preferably from 2 to 60, for example from 2 to 17, or from 18 to 80, for example from 18 to 60.
The starting alkane can also be a substituted cyclic alkane (with one or more rings) corresponding, for example, to the general formula CmH2m in which m is a number ranging from 4 to 60, preferably from 4 to 30. It is a matter in particular of a cycloalkane substituted by at least one linear or branched alkane chain corresponding, for example, to the general formula 
in which x is a number equal to or greater than 2, preferably ranging from 2 to 20, and y is a number equal to or greater than 0, preferably ranging from 0 to 29. In the case where the starting alkane is a substituted cycloalkane, the reaction takes place on the substituted alkane chain of the cycloalkane.
The starting alkane can be used alone or in a mixture with one or more other starting alkanes, such as those described above.
The organometallic compound (B) comprises a metal bonded to at least one hydrocarbon-comprising radical. The metal can be chosen from transition metals, in particular the metals from columns 3, 4, 5 and 6 of the Table of the Periodic Classification of the Elements mentioned above, and from lanthanides and actinides. The metal can, for example, be chosen from scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cerium and neodymium. Preference is given to a metal chosen from the transition metals of the abovementioned columns 4, 5 and 6 and in particular from titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. More particularly, preference is given to tantalum, chromium, vanadium, niobium, molybdenum or tungsten.
The metal of the organometallic compound (B) can be present at any oxidation state. It can be at an oxidation state greater than its minimum or, preferably, be at its maximum oxidation state, in particular when the metal is chosen from transition metals of the abovementioned columns 4, 5 and 6. In this group, for example, chromium and niobium are preferred at an oxidation state ranging from 4 to 6 and vanadium at an oxidation state equal to 4 or 5.
The organometallic compound (B) comprises a metal bonded to at least one hydrocarbon-comprising radical, in particular an aliphatic or alicyclic and saturated or unsaturated hydrocarbon-comprising radical and preferably a C1 to C20, in particular C1 to C10, hydrocarbon-comprising radical.
The metal can be bonded to a carbon of the hydrocarbon-comprising radical via a single, double or triple bond. The bond concerned may in particular be a single carbon-metal bond of a type: in this case, the hydrocarbon-comprising radical is an alkyl radical, for example a linear or branched radical. The term xe2x80x9calkyl radicalxe2x80x9d is understood to mean an aliphatic monovalent radical originating from the removal of a hydrogen atom in the molecule of an alkane or of an alkene or of an alkyne, for example a methyl (CH3xe2x80x94), ethyl (C2H5xe2x80x94), propyl (C2H5xe2x80x94CH2xe2x80x94), neopentyl ((CH3)3Cxe2x80x94CH2xe2x80x94), allyl(CH2xe2x95x90CHxe2x80x94CH2xe2x80x94) or ethynyl (CHxe2x89xa1Cxe2x80x94) radical. The alkyl radical can be, for example, of formula Rxe2x80x94CH2xe2x80x94, where R itself represents a linear or branched alkyl radical.
It can also relate to a carbon-metal double bond of xcfx80 type: in this case, the hydrocarbon-comprising radical is an alkylidene radical, for example a linear or branched radical. The term xe2x80x9calkylidene radicalxe2x80x9d is understood to mean an aliphatic bivalent radical originating from the removal of two hydrogen atoms on the same carbon of the molecule of an alkane or of an alkene or of an alkyne, for example a methylidene (CH2xe2x95x90), ethylidene (CH3xe2x80x94CHxe2x95x90), propylidene (C2H5xe2x80x94CHxe2x95x90), neopentylidene ((CH3)3Cxe2x80x94CHxe2x95x90) or allylidene (CH2xe2x95x90CHxe2x80x94CHxe2x95x90) radical. The alkylidene radical can, for example, be of formula Rxe2x80x94CHxe2x95x90, where R represents a linear or branched alkyl radical.
The carbon-metal bond can also be a triple bond: in this case, the hydrocarbon-comprising radical is an alkylidyne radical, for example a linear or branched alkylidyne radical. The term xe2x80x9calkylidyne radicalxe2x80x9d is understood to mean an aliphatic trivalent radical originating from the removal of three hydrogen atoms on the same carbon of the molecule of an alkane or of an alkene or of an alkyne, for example an ethylidyne (CH3xe2x80x94Cxe2x89xa1), propylidyne (C2H5xe2x80x94Cxe2x89xa1), neopentylidyne ((CH3)3Cxe2x80x94Cxe2x89xa1) or allylidyne (CH2xe2x95x90CHxe2x80x94Cxe2x89xa1) radical. The alkylidyne radical can be, for example, of formula Rxe2x80x94Cxe2x89xa1, where R represents a linear or branched alkyl radical. It is preferable to have, among the alkyl, alkylidene or alkylidyne radicals, in particular the methyl, ethyl, propyl, isobutyl, neopentyl, allyl, neopentylidene, allylidene and neopentylidyne radicals.
The metal of the organometallic compound (B) can be bonded to more than one hydrocarbon-comprising radical. It can in particular be bonded to two or more identical or different hydrocarbon-comprising radicals chosen from alkyl, alkylidene and alkylidyne radicals. In particular, it can be bonded to at least one alkyl radical and to at least one alkylidene or alkylidyne radical. The number of hydrocarbon-comprising radicals bonded to a metal depends on the oxidation state of the metal: it is generally less than the latter and in particular equal to or less than the number of the oxidation state of the metal minus one.
The metal of the organometallic compound (B) which is bonded to at least one hydrocarbon-comprising radical can in addition be bonded to at least one alkoxy radical and/or one halogen atom. The alkoxy radical can correspond to the general formula Rxe2x80x94Oxe2x80x94 in which R represents a linear or branched alkyl radical, for example a C1 to C10 radical. The alkoxy radical can be, in particular, the methoxy, ethoxy, propoxy or butoxy radical. The halogen atom can be fluorine, chlorine, bromine or iodine.
The organometallic compound (B) is fixed to an inorganic support which can be chosen from oxides or sulphides. Preference is given to an inorganic support, such as a metal oxide or refractory oxide, or a mixture of metal oxides, for example silica, alumina, a mixture of silica and of alumina, zeolites, natural clays, aluminium silicates, titanium oxide, magnesium oxide, niobium oxide or zirconium oxide. The inorganic support can be a metal oxide or refractory oxide modified with an acid, such as a sulphated zirconia or a sulphated alumina. The inorganic support can also be a metal sulphide, such as a molybdenum or tungsten sulphide, a sulphurized alumina or a sulphurized metal oxide. It is preferable to use an inorganic support- chosen from porous or non-porous silicas and aluminas, for example mesoporous silicas and aluminas having pores of 20 to 200 xc3x85.
The inorganic support based on metal oxide or refractory oxide has the advantage of exhibiting, at its surface, oxygen atoms which can form part of the coordination sphere of the metal of the organometallic compound (B). Thus, the metal can advantageously be bonded to one or more functional groups of the inorganic support. In this case, if the inorganic support is a metal oxide or refractory oxide, the metal of the organometallic compound can be bonded to one or more oxygen atoms of the inorganic support. The presence of at least one oxygen-metal bond confers greater stability on the organometallic compound (B) while providing a strong support-metal bond.
Thus, in an illustrative fashion, the organometallic compound (B) fixed to an inorganic support can correspond to the following general configuration: 
in which S represents an atom of the inorganic support, M represents the atom of the metal of the compound (B), R represents the hydrocarbon-comprising radical or radicals of the compound (B), which are identical to or different from one another, Y represents an alkoxy radical and/or a halogen atom, x is a number ranging from 1 to 3, preferably equal to 1 or 2, y is a number ranging from 0 to (nxe2x88x92xxe2x88x921), preferably equal to 0, 1 or 2, n is a number representing the oxidation state of the metal M and z is a number at least equal to 1 and less than or equal to (nxe2x80x94xxe2x80x94y). In this definition, the metal, the hydrocarbon-comprising radical, the alkoxy radical and the halogen atom are those described above for the compound (B).
When the inorganic support comprises a metal oxide or refractory oxide, the general configuration (6) can then be written as (7): 
in which configuration M, R, Y, x, y, z and n have the same definitions as above, and Me represents a metal atom or an atom of the main groups of the metal oxide or refractory oxide and 0 the oxygen atom of this same oxide. Thus, when the inorganic support is a silica or a zirconia, the general configuration (7) can then be written: 
in which configurations M, R, Y, O, n, x, y, z and n have the. same definitions as above, and Me represents a silicon or zirconium atom of the inorganic support, with z in particular at least equal to 1 and less than or equal to (nxe2x88x922xe2x88x92y) in the configuration (9).
In the same way, when the inorganic support is an alumina, the general configuration (7) can then be written as (10) or (11): 
in which configurations M, R, Y, O, n, x, y, z and n have the same definitions as above, and Al represents an aluminium atom of the alumina, with z in particular at least equal to 1 and less than or equal to (nxe2x88x922xe2x88x92y) in the configuration (11).
The organometallic compound (B) fixed to an inorganic support can be prepared in various ways. One of the preparation processes can comprise the dispersion and the grafting:
(1) of an organometallic precursor (B) having a metal and at least one hydrocarbon-comprising radical bonded to the said metal which are identical to those of the compound (B) and having in particular the general formula
MRnxe2x88x92yYyxe2x80x83xe2x80x83(12)
xe2x80x83in which M represents the atom of the metal of the compound (B), R represents the hydrocarbon-comprising radical or radicals of the compound (B), which are identical to or different from one another, Y represents an alkoxy radical and/or a halogen atom as defined above for the compound (B), n is a number representing the oxidation state of the metal M and y a number ranging from 0 to (nxe2x88x921), preferably equal to 0, 1 or 2, with (nxe2x88x92y) equal to or greater than 1, over and to
(2) an inorganic support, in particular a metal or refractory oxide, as described above.
The organometallic precursor (E) can itself be prepared by various methods known per se. Thus, for example, it can be prepared by an alkylation reaction of a halide or an alkoxide or an amide of the metal M using an organomagnesium compound or a Grignard.
The attachment or grafting of the organometallic precursor (E) to the inorganic support is preferably carried out by reacting the said precursor with the support and in particular with the functional groups of the support, such as OH groups or oxygen atoms when the support is a metal oxide or refractory oxide. The preparation is preferably carried out under an inert atmosphere, such as nitrogen or argon, and in particular under a non-reducing atmosphere, for example in the absence of hydrogen.
According to a preferred embodiment, the organometallic precursor (E) is grafted to a particularly anhydrous and water-free support. The solid support can be heat-treated beforehand in order to carry out a dehydration and/or a dehydroxylation, in particular at a temperature of 200 to 1100xc2x0 C., for several hours, for example from 2 to 48 hours, preferably from 10 to 20 hours. The maximum temperature of the heat treatment is preferably below the sintering temperature of the support. Thus, for silica, a dehydration and/or a dehydroxylation can be carried out at a temperature of 200 to 500xc2x0 C., for example at approximately 500xc2x0 C., or else at a temperature ranging from 500xc2x0 C. to the sintering temperature of the silica, in order in particular to form siloxane bridges at the surface of the support.
The grafting of the organometallic precursor (E) to the support can be carried out in various ways, in particular by an operation of sublimation of the precursor (E) or by bringing the said precursor into contact with the support in solvent or liquid medium.
In the case of a sublimation operation, the organometallic precursor, used in the solid state, is heated under vacuum and under temperature and pressure conditions which provide for its sublimation and its migration in the vapor state onto the support. The latter is preferably used in pulverulent form or in the form of pellets. The sublimation is carried out in particular between 50 and 150xc2x0 C., preferably between 60 and 100xc2x0 C., under vacuum. In particular, the grafting of the organometallic precursor (E) to the support can be monitored using infrared spectroscopic analysis.
In the method which has just been described, the sublimation can be replaced by an operation in which the organometallic precursor (E) and the support are brought into contact and reacted in solvent or liquid medium. In this case, the organometallic precursor (E) is preferably dissolved in an organic solvent, such as pentane or ether. The reaction is then carried out by suspending the support, preferably in a pulverulent form, in the solution comprising the organometallic precursor (E) or alternatively by any other method which provides contact between the support and the organometallic precursor (E). The reaction can be carried out at room temperature (20xc2x0 C.) or more generally at a temperature ranging from xe2x88x9280xc2x0 C. to +150xc2x0 C. under an inert and preferably non-reducing atmosphere, for example a nitrogen atmosphere.
The excess organometallic precursor (E), which is not attached to the support, can be removed, for example by washing or reverse sublimation.
The process of the invention therefore comprises an alkane cross-metathesis reaction by which the starting alkane (A) is reacted with the organometallic compound (B). The reaction thus makes it possible to convert the starting alkane (A) into at least one of its higher or lower homologues by cleavage and recombination with the hydrocarbon-comprising radical of the organometallic compound (B). The choice of the starting alkane (A) and of the hydrocarbon-comprising radical of the organometallic compound (B) is important, because it directly determines the type of the alkane (C) desired and formed by the cross-metathesis reaction. All the possible combinations can be achieved from all the choices provided above for the alkane (A) and the hydrocarbon-comprising radical of the organometallic compound (B). The number of carbon atoms in the starting alkane (A) can be identical to that in the hydrocarbon-comprising radical of the organometallic compound (3): in this case, the isomeric form of the alkane (A) and the hydrocarbon-comprising radical can be identical or, preferably, different. Generally, it is preferable to choose a starting alkane (A) for which the number of carbon atoms is different from that in the hydrocarbon-comprising radical of the organometallic compound (B).
The cross-metathesis reaction between the starting alkane (A) and the organometallic compound (B) can preferably be carried out by passing the starting alkane in the gas phase over the solid compound (B). The reaction can be carried out at atmospheric pressure or above but at a pressure less than or equal to the condensation pressure of the starting alkane or of the heaviest starting alkane, when there are several starting alkanes. The reaction can also be carried out in the liquid phase in the starting alkane or in a mixture of starting alkanes with the compound (B) in suspension. The reaction can also be carried out in the presence of an inert gas, such as nitrogen, helium or argon.
The cross-metathesis reaction according to the invention can be carried out in a static reactor, that is to say with a fixed amount of reactants introduced for a complete reaction cycle, or in a recycling reactor, in which the alkanes obtained can in particular be recycled, or in a dynamic reactor, that is to say by passing a flow of the liquid or gaseous starting alkane or alkanes over a bed of the compound (B).
The cross-metathesis reaction can be carried out at temperatures varying from 20 to 400xc2x0 C., preferably from 100 to 300xc2x0 C., under an absolute pressure which can range from 10xe2x88x923 to 10 MPa.
The process of the invention can comprise a recycling of the alkanes obtained during the reaction. It can relate equally well to the recycling of a specific alkane and to the recycling of several alkanes, in order to continue the reaction towards the production of desired alkanes. It is optionally possible to provide for separation between several alkanes, for example with the intention of recycling one alkane or another.
The cross-metathesis reaction can be carried out at temperatures varying from 20 to 400xc2x0 C., preferably from 100 to 300xc2x0 C., under an absolute pressure which can range from 10xe2x88x923 to 10 MPa.
The process of the invention can comprise a recycling of the alkanes obtained during the reaction. It can relate equally well to the recycling of a specific alkane and to the recycling of several alkanes, in order to continue the reaction towards the production of desired alkanes. It is optionally possible to provide for separation between several alkanes, for example with the intention of recycling one alkane or another.
According to another aspect of the present invention, the process for the manufacture of alkanes can comprise one or more stages additional to the main stage. The additional stage can be carried out before or after the main stage. The additional stage can consist essentially:
either of a cross-metathesis reaction other than that of the main stage, employing, for example, at least one starting alkane (A) other than that used in the main stage, or else a different organometallic compound (B), in particular comprising at least one hydrocarbon-comprising radical other than that of the compound (B) used in the main stage, or else alternatively, simultaneously, at least one starting alkane (A) and a compound (B) which are other than those in the main stage,
or of a conventional alkane metathesis reaction employing at least one starting alkane identical to or different from that used in the main stage but brought into contact with a solid catalyst comprising a metal hydride of a metal, identical to or different from the metal of the organometallic compound (B) of the main stage, chosen in particular from the metals of groups 5 and 6 of the abovementioned Table of the Periodic Classification of the Elements, in particular with an oxidation state less than its maximum value, the metal hydride being in particular grafted to and dispersed over a solid support based on metal oxide or refractory oxide, in particular as disclosed in Patent Application PCT/FR 97/01266.
When the additional stage is carried out before the main stage, the flow of the products in particular of the alkane or alkanes resulting from the additional stage, can be partially or completely used as starting material(s) (that is to say, as starting alkane(s) (A)) in the main stage according to the present invention.
When, in contrast, the additional stage is carried out after the main stage, the flow of the products resulting from the main stage according to the present invention, in particular the flow of the other alkane or alkane(s) (C), can be partially or completely used as starting material(s) (that is to say, as starting alkane(s)) in the additional stage.
The flow of the products resulting from one stage can in particular be directed continuously to the other stage, in particular when the main stage and the additional stage are themselves carried out continuously.
The process according to the present invention can be carried out continuously, in particular by a continuous addition of the starting alkane (A) to the organometallic compound (B), in particular when the latter is used in the solid state in a pulverulent form, for example in the form of a fixed or fluidized bed. The process can also be carried out continuously by continuous and simultaneous addition of the starting alkane (A) and of the organometallic compound (B), in particular when the latter is used in the form of a suspension in a liquid.